Position sensing of a remote target

ABSTRACT

Apparatus for the position sensing of a remote target. A preferred embodiment is used to control the position of a graphical pointer on a computer screen. A light emitting diode illuminates the head of the computer operator and an array of detectors detect radiation returned by a retroreflector attached to the head of the operator. Signal processing means is included to derive information about the position of the operator&#39;s head from the detector outputs and convert it to a form suitable for input to computer.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The current invention is concerned with position sensing of a remotetarget and has particular, but not exclusive, relevance to the computerindustry.

2. Discussion of Prior Art

The computer industry is a rapidly evolving area of technology wheremanufacturers are constantly seeking innovations which will provide anadvantage over competitors products. One particular area which is theobject of much attention is the Human Computer Interface (HCI) or ManMachine Interface, that is, the means by which the computer operatorcommunicates with the computer, inputting data and commands andreceiving information.

The use of menu driven and windowing software greatly enhances the userfriendliness of many computer related products, especially if suchsoftware is used in conjunction with a computer mouse or trackerball.These are handheld devices which allow the user to interact withsoftware/hardware by moving a graphical pointer around on a computerscreen, making selections from displayed options, manipulating softwarewindows, drawing etc. by moving the mouse over a surface or operatingthe tracker ball by thumb motion and taking action by depressing abutton.

A particular application of the current invention offers a system wherethe handheld mouse is redundant and the pointer on screen moves inresponse to movements of the operators head. This leaves both hands freeto operate the keyboard which is particularly advantageous to, forexample, typists.

The control of the cursor by movements of the operators head is veryergonomic and intuitive. The equivalent to depressing the mouse buttonmay be achieved either by additional buttons on the keyboard or bydetection of specific head movements such as a shake or nod. This may beachieved without the need for any wires, batteries or other impedimentaon the operator and may be implemented at a cost comparable to theexisting mouse.

There are various ways in which the motion of the operators head can beused to control the cursor position on the screen.

Firstly, the motion of the head and the cursor may be locked togetherexactly, regardless of the speed of motion of the head. This is anabsolute mode and is used in the implementation described in thisspecification.

Secondly, the motion of the head and cursor may be coupled in a mannerdependent on both the position and speed of motion of the head. This isa relative mode and is the speed sensitive option available with somemouse systems. Slow movement of the head by one cm corresponds tomovement of the cursor by one centimeter while rapid movement of thehead by one cm results in say four cm of cursor movement. This optionwould be available with the system described and could be implemented insoftware or hardware.

Thirdly, the cursor may move only in response to motion of the head andnot the absolute position of the head. The absolute head position can beextracted when a signal is present, but that signal is only presentduring motion.

Many other applications of the invention are envisaged: by exploitingthe retroreflecting properties of the human eye, eye movements may besensed to ensure that the driver of a vehicle is not falling asleep; theinvention may be used to track objects other than the operators head; bymovement of hand or foot or limb stump, disabled persons may controlsoftware driven apparatus carrying out any number of tasks; eye movementand head movement scanned in combination will allow gaze directioninformation to be extracted; in industry, process control in a dirtyenvironment could be effected with the hands free; a pair systems of theinvention could be used to obtain two directional fixes on a target,allowing the position in three dimensions to be established.

SUMMARY OF THE INVENTION

The invention makes use of electromagnetic radiation emanating from anobject in deriving information about the position of that object. Suchradiation may originate from a source fixed in relation to the object,may be reflected off the object, or the object itself could be thesource of radiation. For the purpose of this specification. "emanatingfrom the object should be taken to include any of these cases.

The radiation emanating from the object is used to illuminate an area ofa plane. This may be achieved, for example, by gathering some of theradiation using a lens or by using a pinhole. By "pinhole" is meant anaperture through which a beam of radiation can pass, said beam having adefined boundary or boundaries and having sufficient intensity for theeffective operation of the invention. Then, by the parallax effect, thetwo dimensional position of the illuminated area is dependent on theangular position of the target.

According to this invention, apparatus for the position sensing of aremote target comprises:

a source of radiation;

an array of detector elements, each of which is independently capable ofproducing an electrical output which is dependent on the intensity ofradiation incident thereon and

signal processing means for deriving information about the angularposition of the target from the outputs of said detector elements,

and is characterized by:

a retroreflector whose position is fixed in relation to the target andin that the source of radiation is located substantially on a line onwhich lies the array of detector elements and said retroreflector, saidsource being arranged to illuminate said retroreflector such thatradiation is retroreflected towards the array and the intensity ofradiation incident on each detector element varies, by the parallaxeffect, with the angular position of the target.

A preferred embodinent further includes a pinhole, located substantiallyon a line on which lies the array of detector elements and the source ofradiation, and through which retroreflected radiation passes beforeilluminating said array.

The apparatus may be used to control the position of a graphical pointeron the computer screen. In such an embodiment the target might be partof the computer operator's body.

The detector elements could each comprise a photodiode.

Filtering means might be included for preventing radiation of adifferent wavelength to that emanating from the source from illuminatingsaid array.

The radiation emanating from the source could be in the visible ofinfrared regions of the electromagnetic spectrum, or in some otherregion.

The apparatus might include means for modulating the output of theradiation source and corresponding means for demodulating the output ofthe detectors.

The general principles of the current invention may be applied to theposition sensing of a remote target in general. In situations where thesource of radiation is the target itself (for example infrared emissionfrom the human body), passive systems are envisaged which could beapplied, for example, in security systems.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will now be described with reference to the followingfigures in which:

FIG. 1 shows part of an embodiment of the invention in which the sourceof radiation is fixed to the operators head;

FIG. 2 shows part of an embodiment of the invention which utilises afluorescent label, used in conjunction with a remote source ofradiation, to provide radiation emanating from the direction of theoperators head;

FIG. 3 shows part of an embodiment of the invention which uses aretroreflector used in conjunction with a source of radiation to provideradiation emanating from the direction of the operators head;

FIG. 4 shows electronic circuitry used to process the outputs of thedetector elements;

FIG. 5 shows further processing circuitry used to derive information, inone dimension, about the position of the target and

FIG. 6 is a representation of the interface between the currentinvention and a personal computer.

DETAILED DISCUSSION OF PREFERRED EMBODIMENTS

Referring to FIG. 1, a simple embodiment of the invention comprises asource of electromagnetic radiation 1, for example a light emittingdiode, attached to the head of the operator 2. Source 1 could, forexample, take the form of a penlight tucked behind the ear. Radiationfrom source 1 is gathered by lens 3 and concentrated on an illuminatedarea 4 of a forward scattering diffuser screen 5 which could be a sheetof ground glass or greaseproof paper.

A backward scattering diffuser screen may prove equally effective (forexample a sheet of white paper). In this case, the scattered light wouldbe reflected and diodes 6a-6d would be situated on the opposite side ofthe screen 5 to that shown.

An array of electromagnetic radiation detector elements 6a, 6b, 6c and6d, conveniently located in the vicinity of computer screen 7, detectsradiation from illuminated area 4. The output of each detector 6a-6ddepends on the position, in two dimensions, of illuminated area 4. Thetwo dimensional position of illuminated area 4 depends, in turn, on theangular position of source 1, controlled by movements of the head of theoperator 2.

In practice, a 2×2 array of detector elements was used, but arrays ofother dimensions may also serve adequately. Throughout thisspecification the term array should be taken to include arrays of alldimensions including 1×1.

Inclusion of optical filter 8, whose passband is based on the wavelengthof radiation produced by source 1, allows discrimination againstradiation from other sources and hence improves the performance of thesystem.

Typically, detector elements 6a-6d would take the form of photodiodes.The use of other types of photodetector, for example phototransistors,is also feasible.

As an alternative to an array of detectors, a position sensitivedetector such as device S1300 available from Hamamatsu Photonics UK Ltdmay be used to derive an electrical signal which is dependent on theposition of illuminated area 4.

Referring to FIG. 2, a fluorescent marker 9 is attached to the head ofthe operator 2. The marker 9 may, for example, be incorporated into apair of spectacles or other article worn by the operator. Radiationsource 1 is placed so that the head of the operator 2, and in particularmarker 9, is within its field of illumination.

During operation marker 9 is illuminated by source 1, and produces, byfluorescence, an emission of radiation. Radiation from marker 9 isgathered by lens 3 and concentrated on an illuminated area 4 of diffuserscreen 5. As before, detector elements 6a-6d detect radiation from area4 and produce an output dependent on the position of the operators head2.

By using a fluorescent marker 9 and an optical filter 8 whose passbandis based on the radiation produced by fluorescence, it is possible todiscriminate against inter alia radiation produced by source 1 andreflected off objects other than the marker on the operators head 2.

Referring to FIG. 3, illumination is supplied by a single Light EmittingDiode (LED) 1 situated substantially on the line between the head of theoperator 2 and the array of detectors 6a-6d and arranged so that thehead of the operator 2 lies within the field of illumination. The LEDused in this particular embodiment was a Siemens SFH 487 GaAs infraredemitter nominally operating at 880 nm producing a 40 degree cone angleof illumination.

Modulation of the output from LED 1 is achieved by means of a 4 kHzsquare wave driving current, typically switching from 0 and 100 mA.Modulation is used to allow measurement of illumination originating fromthe LED 1 whilst discriminating against DC and mains derived frequencycomponents of the background illumination.

A retroreflector 10 is attached to the head of the operator 2 in orderto provide a strong return of radiation in the direction of LED 1. Aperfect retroreflector would return narrow beam of parallel light whichwould be blocked by LED 1 itself. For the purpose of this invention an"imperfect" retroreflector returning a beam in a narrow cone angle ofabout 2 degrees is most suitable and for the purpose of thisspecification the term "retroreflector" should be taken to mean anydevice providing sufficient return of radiation for operation of theinvention.

Some of the reflected radiation passes LED 1 and, after passing throughoptical filter 8 (optional), falls on diffuser screen 5 to produce anilluminated area 4. Diffuser screen 5 is located behind LED 1 so, by theparallax effect, the two dimensional position of illuminated area 4 isdependent upon the angular position of retroreflector 10 and hence theoperators head 2. Therefore photodiodes 6a-6d each produce an outputwhich is dependent on the angular position of the operators head 2. Thesystem may be improved by inclusion of a lens 3 to gather returnedradiation and focus it into a more concentrated image on screen 5.

Also, it is feasible that screen 5 may be omitted so that radiationreturned from retroreflector 10 falls directly on to the array ofdetectors 6a-6d producing an area of illumination there.

Referring to FIG. 4, the circuitry represented therein is replicated foreach of the four photodiodes 6a-6d shown in FIGS. 1-3.

The signal from each photodiode 6a-6d passes through a current tovoltage converter stage 11. The preferred option for amplifier 12 usedtherein is a low noise operational amplifier with wide dynamic range.Where modulation of the illuminating radiation is used, a high passfilter 13 is used to discriminate against AC, DC and noise components ofthe signal which are due to background illumination. The cutoff of thefilter used represents a balance between rejection of said AC, DC andnoise components on the one hand and avoiding phase shifting of themodulated signal on the other. The latter gives rise to errors whensumming the signals from each photodiode. With the 4 kHz modulationfrequency associated with the apparatus of FIG. 3, a 2 kHz cutofffrequency was used.

The demodulated signal is then rectified and integrated at stage 14.

In order to reduce the dependence of the system on the distance of theoperators head, automatic gain control of the signal is achieved bysumming the four rectified, demodulated signals respectively associatedwith the four photodiodes 6a-6d and dividing each signal by the sum. Thesummation takes place at stage 15 which includes an offset voltage inorder to limit the gain.

The individual and summed signals are each fed to stage 16 comprising anAD534 device supplied by Analogue Devices) where automatic gain controlis effected.

Referring to FIG. 5, the horizontal signal is derived 17 by combiningthe four range compensated signals to obtain the quantity (v_(a)+v_(c))-(v_(b) +v_(d)), where v_(a), v_(b) etc. are the rangecompensated signals respectively derived from photodiodes 6a, 6b etc.

The magnitude of the systems response to lateral movements of theoperators head may be adjusted at output limiter/sensitivity controlstage 18.

A similar arrangement to that shown in FIG. 5 is used to derive thevertical signal by combining the four range compensated signals toobtain the quantity (v_(a) +v_(b))-(v_(c) +v_(d)).

Referring to FIG. 6, the analogue outputs, from stage 18 FIG. 5,representing horizontal and vertical displacement are each converted toa 12-bit digital value by a two channel, 12 bit analogue to digitalconverter 19. The use of 12 bits permits a resolution of 4096 in eachaxis which is adequate for translation into personal computer screencoordinates of about 1024 pixels in each axis. The digitized horizontaland vertical displacement values are further processed by serialprotocol conversion hardware 20 to provide an encoded RS232 sequencecombining the displacement values and the status of a number ofadditional inputs 21 (in this case two) which take the place of themouse buttons. In this embodiment, these inputs were derived fromcontact switches attached to the computer keyboard. An alternativeembodiment takes action, as if a conventional mouse button has beenpressed, in response to specific head movements (eg a nod or shake).

An implementation of the serial protocol conversion hardware 20 has beenconstructed using analogue to digital convertors and programmable logicarrays. However, other means for meeting the requirements specifiedabove will be apparent to those skilled on the art.

The software package used was Windows (TM) version 3.1, produced byMicrosoft (RTM) Corporation, run on a Compaq (TM) SLT personal computer.The Windows (TM) mouse device driver software was modified to accept thehorizontal and vertical displacement information along with theinformation concerning the status of the additional inputs via theserial interface of the personal computer 22. This information isreceived on an interrupt basis. Each horizontal, and each vertical,datum received is relative to the last and the mouse driver softwaretracks this information passing an appropriate scaled and signed datumto the windows software. Thus the driver clips the coordinates of theremote position sensor to the actual screen coordinates.

The information from the additional inputs 21 corresponding to the leftbutton of a conventional mouse was used as for a conventional mouse,that is to "click" on software options displayed on the computer screen.The other switch is used to reset the coordinates to the middle of thescreen.

We claim:
 1. Apparatus for controlling the position of a graphicalpointer on a screen comprising:a source of electromagnetic radiation, aradiation retroreflector, an array of at least two detector elements fordetecting radiation emitted by the source and reflected by theretroreflector, each detector element producing an output which isdependent on the intensity of radiation incident thereon, and processingmeans for determining an angular position of the retroreflector from theoutputs of the detector elements, a drive circuit, responsive to saidprocessing means, for generating a graphical position control signal forsaid screen wherein the source of radiation is positioned on an axiswhich is substantially central and normal to the array such thatradiation reflected by the retroreflector passes the source to bedetected by the array and such that illumination of the array isdependent on the angular position of the retroreflector with respect tothe axis, and wherein the retroreflector has a return beam cone anglesuch that sufficient reflected radiation passes the source to bedetected by the array, further comprising a diffuser screen locatedoptically between the source and the array.
 2. Apparatus according toclaim 1 wherein the apparatus further comprises a lens for focusingradiation reflected by the retroreflector.
 3. Apparatus according toclaim 1 wherein the return beam cone angle is approximately two degrees.4. Apparatus for controlling the position of a graphical pointer on ascreen comprising:a source of electromagnetic radiation, a radiationretroreflector, a detector element for detecting radiation emitted bythe source and reflected by the retroreflector, the detector elementproducing an output which is dependent on the position of radiationincident thereon, processing means for determining an angular positionof the retroreflector from the output of the detector element, and adrive circuit, responsive to said processing means, for generating agraphical position control signal for said screen wherein the source ofradiation is positioned on an axis which is substantially central andnormal to the detector element such that radiation reflected by theretroreflector passes the source to be detected by the detector elementand such that illumination of the detector element is dependent on theangular position of the retroreflector with respect to the axis, andwherein the retroreflector has a return beam cone angle such thatsufficient reflected radiation passes the source to be detected by thedetector element further comprising a diffuser screen located opticallybetween the source and the detector element.
 5. Apparatus according toclaim 4 wherein the apparatus further comprises a lens for focusingradiation reflected by the retroreflector.
 6. Apparatus according toclaim 4 wherein the return beam cone angle is approximately two degrees.7. An apparatus permitting an operator to control the position of agraphical pointer on a screen, said apparatus comprising:a source ofelectromagnetic radiation along an axis; an array of at least twodetector elements centrally located on said axis, each element producingan output indicative of the intensity of said radiation incidentthereon, a retroreflector of said radiation for reflecting at least aportion of said radiation on said array, said source located betweensaid array and said retroreflector, said retroreflector having anangular position controlled by an operator, wherein changes in saidretroreflector angular position by said operator changes position ofsaid radiation on said array; and a processor, responsive to saiddetector element outputs, for providing a position controlling signalfor said graphical pointer.
 8. An apparatus according to claim 7,wherein said array comprises four elements, said four elements locatedin a plane normal to said axis.
 9. An apparatus according to claim 7,wherein said retroreflector has a return beam cone angle of about 2degrees.
 10. An apparatus according to claim 7, further including adiffuser screen located between said source and said array.
 11. Anapparatus permitting an operator to control the position of a graphicalpointer on a screen, said apparatus comprising:a source ofelectromagnetic radiation along an axis; a detector centrally located onsaid axis, said detector producing an output indicative of the positionof said radiation incident thereon, a retroreflector of said radiationfor reflecting at least a portion of said radiation on said detector,said source located between said detector and said retroreflector, saidretroreflector having an angular position controlled by an operator,wherein changes in said retroreflector angular position by said operatorchanges position of said radiation on said detector; and a processor,responsive to said detector output, for providing a position controllingsignal for said graphical pointer.
 12. An apparatus according to claim11, wherein said detector is located in a plane normal to said axis. 13.An apparatus according to claim 11, wherein said retroreflector has areturn beam cone angle of about 2 degrees.
 14. An apparatus according toclaim 11, further including a diffuser screen located between saidsource and said detector.
 15. A method of controlling the position of agraphical pointer on a screen, said method comprising the stepsof:providing electromagnetic radiation from a source and directed in acone along an axis; controlling the angular position of a retroreflectorlocated within said cone, to at least partially reflect said radiation;detecting the position of said reflected radiation relative to said axisat a distance from the retroreflector greater than the distance from theretroreflector to said source of radiation; and providing, in responseto the detected position, a position controlling signal for saidgraphical pointer.